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OCR Chemistry Unifying Concepts 11th June 2009

Can anyone give me a hand,this is a really easy question but I just cannot get my head around it..Anyone want to have a go at explaining to me how to figure out the concentration?
It's Jan 04,Question (2),part (d),part (ii)

Reply 41

JUSTME1

well 20 volume h202 means for every volume of h2o2 20 volume 02 released

1dm-3 will release 20dm-3 of oxygen

equation is:

H202................. h20 + 0.5 02

so we know 1dm-3 of h2o2 releases 20dm-3 of oxygen

moles of oxygen is volume divided by 24dm

20/24 = 0.83 times ten to the power minus 1

you have to times the moles of oxygen by 2 as mole ratio is 0.5:1

so it is 2 x 20 / 24 =1.67

hope that helps

Reply 42

kat_ie

i'm really not ready for this exam, we haven't even finished the lessons yet...

Reply 43

JamesssG

Above... oh dear. Guys this is simple if you read the spec on what you know. Our rubbish text book tells us lots we don't need to know. Tomorrow I shall be uploading a revision sheet with all we need to know. I've found that by going through my revision sheet, reading one topic say How Fast? Find questions on it, and doing them till its in my head, then going on to they are all done has helped loads. I now get all of it, (although most of it is v.similar). Exam layout is usually 3 or 4 questions from the main topics (How Far? [rates] How Fast [equilibrium] and Acid, Bases & Buffers.) Sometimes (well normally) there is a long answer at the end for everything, but if Maths is your subject. You'll ace this. Trends and Patterns also is a simple one. Biochem however is hard if like me, you don't do Biology.

Reply 44

Shavaunt

2

Biochemistry is my worse mainly because I find it so boring (that and I have a passionate hate of biology and biogists hehe :P). But the stuff is not that tricky... well okay DNA is tricky but biochem is just organic chemistry but more focused.

Reply 45

i_hate_examz

10

JamesssG

Above... oh dear. Guys this is simple if you read the spec on what you know. Our rubbish text book tells us lots we don't need to know. Tomorrow I shall be uploading a revision sheet with all we need to know. I've found that by going through my revision sheet, reading one topic say How Fast? Find questions on it, and doing them till its in my head, then going on to they are all done has helped loads. I now get all of it, (although most of it is v.similar). Exam layout is usually 3 or 4 questions from the main topics (How Far? [rates] How Fast [equilibrium] and Acid, Bases & Buffers.) Sometimes (well normally) there is a long answer at the end for everything, but if Maths is your subject. You'll ace this. Trends and Patterns also is a simple one. Biochem however is hard if like me, you don't do Biology.

That would be great ! :smile:

Reply 46

JamesssG

its almost complete, im just doing some acid buffer and base questions to make sure i full understand :smile:

Reply 47

Ciaran

13

JamesssG

its almost complete, im just doing some acid buffer and base questions to make sure i full understand :smile:

nice one, so whats in the revision sheet then? just everything in the unifying syllabus?

Reply 48

JamesssG

Yeh my mahn! Everything we need know from spec.

Reply 49

Ciaran

13

JamesssG

Yeh my mahn! Everything we need know from spec.

nice, you gonna upload them soon?

Reply 50

Bassist Jonathan

I think some people may have the wrong grasp of buffer solutions so here is a quick summary:

An acidic buffer consist of a weak acid and the salt of a weak acid (eg. CH3COOH and CH3COO- ) and minimises the change in pH with addition of an acid or a base.

So in a question you would write down this equation:

CH3COOH <--> CH3COO- + H+

If a small amount of acid is added (H+) equilibrium shifts to left.

If a small amount of base is added (OH-) equilibrium will shift to the right.*

These 4 simple points will get you at least 4 marks and are really easy to get your head around.

Now theres often other marks for using different acids and bases that they provide you with but always remember the same principles.

*Its worth noting OH- causes a shift to the right because it reacts with the H+ on the CH3COOH meaning that there is a lack of H+

Reply 51

JamesssG

Uploading in about 10 mins my friends.

Reply 52

JamesssG

http://www.freewebs.com/clanvi/Chemistry/Unifying%20Concepts.doc

There we go, hope it helps :smile:

Reply 53

i_hate_examz

10

JamesssG

http://www.freewebs.com/clanvi/Chemistry/Unifying%20Concepts.doc

There we go, hope it helps :smile:

Thanks Mate !!!! (reppp)

<3

cheers mate !

OP

Bassist Jonathan

I think some people may have the wrong grasp of buffer solutions so here is a quick summary:

An acidic buffer consist of a weak acid and the salt of a weak acid (eg. CH3COOH and CH3COO- ) and minimises the change in pH with addition of an acid or a base.

So in a question you would write down this equation:

CH3COOH <--> CH3COO- + H+

If a small amount of acid is added (H+) equilibrium shifts to left.

If a small amount of base is added (OH-) equilibrium will shift to the right.*

These 4 simple points will get you at least 4 marks and are really easy to get your head around.

Now theres often other marks for using different acids and bases that they provide you with but always remember the same principles.

*Its worth noting OH- causes a shift to the right because it reacts with the H+ on the CH3COOH meaning that there are more CH3COO-

Right, I see what you're saying. But surely the equilibrium shifts to the right because of the lack of H+ since its reacted with the OH-? Doesn't really matter theres more CH3COO-.